Hydroxyapatite from Fish for Bone Tissue Engineering: A Promising Approach

Natural or synthetic hydroxyapatite (HA) has been frequently used as implant materials for orthopaedic and dental applications, showing excellent bioactivity, adequate mechanical rigidity and structure, osteoconductivity and angiogenic properties, no toxicity, and absence of inflammatory or antigenic reactions. HA can be easily synthesized or extracted from natural sources, such as bovine bone. However, the manufacturing costs to obtain HA are high, restricting the therapy. Herein, much effort has been paid for obtaning alternative natural sources for HA. The potential of HA extracted from skeleton of animals has been investigated. The aim of this review is to exploit the potential of HA derived from fish to fulfill biological activities for bone tissue engineering. In particular, HA from fish is easy to be manufactured regarding the majority of protocols that are based on the calcination method. Furthermore, the composition and structure of HA from fish were evaluated; the biomaterial showed good biocompatibility as a result of non-cytotoxicity and handling properties, demonstrating advantages in comparison with synthetic ones. Interestingly, another huge benefit brought by HA from bone fish is its positive effect for environment since this technique considerably reduces waste. Certainly, the process of transforming fish into HA is an environmentally friendly process and stands as a good chance for reducing costs of treatment in bone repair or replacement with little impact into the environment.

In this context, the development of alternatives for effective bone reconstruction is one of the most clinically important long-term goals of research within the field of mineralized tissues.
Gene therapy, tissue engineering and biomaterials have emerged as promising approaches for bone regeneration (3).
Biomaterials induce active biomineralization since they are able to induce bone growth (4).
Therefore, clinical regenerative medicine has employed a number of materials, from natural to synthetic origins, in which the so-called bioactive materials are preferred due to their capacity to chemically adhere to bone tissue (4). Bioceramics represent a broad range of inorganic/non-metallic compositions including hydroxyapatite (HA), bioactive glass, calcium phosphate, and others (5).
These materials have been extensively used in clinical practice, acting as fillers and scaffolds for bone reconstruction. Moreover, by promoting a tridimensional matrix that stabilizes and maintains the shape of the filled area, these biomaterials also support cell migration and angiogenesis, resulting in new bone formation during tissue repair (6).
During the last decades, many efforts have been devoted for obtaning alternative natural sources for biomaterials. Herein, the potential of HA extracted from skeleton of animals, including fishes, has been investigated (7). This is because million tons of fish are caught each year for human consumption worldwide (8). However, only 50 to 60% of the total catch is used; while the rest is discarded (9). It is important to stress that there is no commercial viable application for the use of the residues generated from fish markets around the world. Therefore, huge amounts of raw materials are wasted, culminating in undesirable environmental consequences. Moreover, fish waste is destined to landfill, which can cause serious problems to the human health (10). Thus, the conversion of waste obtained from fishing activity into biocompatible materials such as HA to be used for bone tissue engineering aplications, is an encouraging strategy that will minimize the environmental impact, and will incorporate more commercial value for this one. Therefore, the process of transforming waste bone fish into HA is an environmentally friendly process and stands as a good chance of reducing costs for treating bone repair with little impact into the environment (11).
The aim of the review was to present if HA derived from fish was able to fulfill biological activities for bone tissue engineering.
Natural or synthetic HAs have been frequently used as a material for bone tissue engineering mainly due to their close composition similarity with natural bone (16,17). It has been widely investigated as implant material for orthopaedic and dental applications, showing excellent bioactivity, adequate mechanical rigidity and structure, osteoconductivity and angiogenic properties, no toxicity, and absence of inflammatory or antigenic reactions (18). It is well known that, bone tissue binds directly to HA through a carbonated calcium deficient apatite layer at the bone-implant interface, inducing the deposition of newly formed bone after implantation (19). It has been demonstrated that HA surface supports osteoblastic cell adhesion, growth, and differentiation, and newly formed HA is deposited by the creeping substitution from the adjacent living bone (20). One of the advantages of HA is that its microstructure can be controlled to promote the formation of pores that allow the migration of blood vessels and bone tissue into the material (21). Considering the positive osteogenic effects of synthetic HA, some limitations merits attention, especially related to the high manufacturing costs (27). To overcome this problem, HA manufactured from natural sources rather than bovines may be a promising alternative for clinical applications.
Natural materials are often more biocompatible since they offer a better interactive surface for cell attachment and growth (28). In fact, some authors stated that natural HA has better metabolic activity, and more dynamic response to the environment than the synthetic one (29).

HA from fish: extraction and characterization
In the last years, HA from fish bone and scales has emerged as an alternative to substitute synthetic and bovine HA, because similar chemical properties have been achieved by simple and inexpensive methods (10,11). It has been demonstrated that fish sources are safe and present low risks of disease transmission (10). Additionally, fishes are abundant in the environment, and the application of their byproducts is suitable for biomedical application since it would reduce environmental pollution and threats of biohazards to humans (11). Many different fish species have been used to obtain HA, such as salmon (10) (42,43).
In this context, HA from fish is a promising resource for bone tissue engineering with commercial interest, and for medical and dental products ( Figure 2). However, some limitations and challenges for their use should be overcome.
Among them, all biocompatibility tests should be conducted through in vitro and in vivo studies Recent studies have demonstrated that nano-sized HA can mimic better dimensions the components of bone tissue (47). Thus, nano-HA fish-based biomaterials may present the same advantages of nano synthetic HA, offering a better bioactivity and dissolution than coarser crystals due to large surface to volume ratio and unique chemical properties (48). Therefore, it is expected that nano-HA from fish would promote increased osteoblast adhesion and cell proliferation (49).

Conclusion
In this review, we present all published results reporting the use of HA from fish for tissue engineering. As stated above, HA derived from bone fish is easy to be manufactured, and present good biocompatibility as a result of non-citotoxicity and handling properties, demonstrating advantages in comparison to synthetic HA. Another huge benefit brought by bone fish HA is its positive effect for environment, because the procedure potentially reduces waste. However, further studies are welcomed in light of biocompatibility and biological effects especially using in vivo models before this biomaterial can be used with safety in the clinical setting ( Figure 2). Therefore, this is an area that warrants further investigation to ensure if HA from fish can be better employed, resulting not only in the aggregation of commercial value to the waste produced by humans, but also as a great benefit, which is a cleaner environment in the future.

Conflict of Interest
None declared.